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Home Interviews How do you validate file format based on file signature (magic number) instead of just file extensio – MindStick
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ICSM Computer
ICSM Computer 14 May 2025
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Validating a file's format using its signature (magic number) is much more reliable than checking the file extension, because it inspects the actual byte content of the file header.

What is a File Signature / Magic Number?

A magic number is a unique sequence of bytes at the beginning of a file that identifies its type.

File Type Signature (Hex) ASCII Example
PDF 25 50 44 46 %PDF
PNG 89 50 4E 47 0D 0A 1A 0A .PNG....
JPG FF D8 FF  
ZIP 50 4B 03 04 PK..
DOCX 50 4B 03 04 ZIP container

How to Read and Validate Signature in C#

using System;
using System.IO;

public class FileSignatureValidator
{
    public static bool IsPdf(string filePath)
    {
        byte[] pdfSignature = { 0x25, 0x50, 0x44, 0x46 }; // %PDF
        return HasSignature(filePath, pdfSignature);
    }

    public static bool IsPng(string filePath)
    {
        byte[] pngSignature = { 0x89, 0x50, 0x4E, 0x47 };
        return HasSignature(filePath, pngSignature);
    }

    public static bool HasSignature(string filePath, byte[] expectedSignature)
    {
        if (!File.Exists(filePath))
            return false;

        using (var stream = File.OpenRead(filePath))
        {
            byte[] buffer = new byte[expectedSignature.Length];
            int read = stream.Read(buffer, 0, buffer.Length);

            if (read != expectedSignature.Length)
                return false;

            for (int i = 0; i < expectedSignature.Length; i++)
            {
                if (buffer[i] != expectedSignature[i])
                    return false;
            }

            return true;
        }
    }
}

Usage:

if (FileSignatureValidator.IsPdf("document.pdf"))
    Console.WriteLine("This is a PDF file.");
else
    Console.WriteLine("Invalid PDF.");

Tips:

  1. For formats like .docx or .xlsx, check the ZIP signature and inspect internal files (like [Content_Types].xml) for deeper validation.
  2. For larger sets of formats, you can build a dictionary of known signatures.
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